1. Field of the Invention
The present invention relates generally to conversion assemblies for use on tilt rotor aircraft for converting from a helicopter mode to an airplane mode, and vice versa. In particular, the present invention relates to a method and apparatus for stabilizing the articulating rotor portion relative to the stationary structure of the aircraft while in the airplane mode.
2. Description of Related Art
Tilt rotor aircraft are hybrids between traditional helicopters and traditional propeller driven aircraft. Typical tilt rotor aircraft have rotor systems that are capable of articulating relative to the aircraft fuselage. This articulating portion is referred to as a nacelle. Tilt rotor aircraft are capable of converting from a helicopter mode, in which the aircraft can take-off, hover, and land like a helicopter; to an airplane mode, in which the aircraft can fly forward like a fixed-wing airplane.
The design of tilt rotor aircraft poses unique problems not associated with either helicopters or propeller driven aircraft. In particular, certain static and dynamic loads are generated by the tilt rotor assemblies that are not present in either conventional helicopters or fixed wing aircraft. While in the aircraft mode, aircraft stability is maintained by a support assembly referred to as a "downstop" assembly. The downstop assembly has two main purposes. First, the downstop assembly must provide vertical stiffness in order to react against the downward forces required to keep the nacelle from rising throughout the flight envelope. Second, the downstop assembly must provide enough lateral stiffness to ensure flight stability. The exact amount of lateral stiffness is based upon aircraft geometry, flight envelope requirements, adjacent part stiffness, and several other factors that are unknown until flight testing is underway. Therefore, it is desirable that the downstop assembly be tunable in such a way that redesign of adjacent parts is not required as a result of the need to increase or decrease the lateral stiffness. If the lateral stiffness is matched or tuned to a particular aircraft's minimum lateral stiffness requirement, then the aircraft's wing structure can be isolated from damaging lateral static and oscillatory loads.
Certain attempts have been made to isolate the static and dynamic loads created between the wing structure and the nacelle while the tilt rotor aircraft is in the airplane mode. In some tilt rotor aircraft, the lateral loads have been isolated by a downstop assembly having long vertical blade. In this application, the height of the vertical blade requires a large fairing to be used, thus increasing the frontal drag of the aircraft. Other tilt rotor aircraft have minimized the height of the downstop assembly, but at the cost of introducing lateral loads into the wing structure. Thus, although great strides have been made in the design of tilt rotor aircraft, the problem of isolating lateral nacelle loads from the wing structure by using a package that is small, adjustable, and vertically stiff has not been adequately resolved.